Ionographic marking systems are disclosed in commonly assigned U.S. Pat. Nos. 4,584,522 and 4,719,481. In each, a fluid jet assisted ion projection marking device places imaging charges upon a moving receptor surface, such as paper, by means of a linear array of closely spaced minute air "nozzles". The charge, comprising ions of a single polarity (preferably positive), is generated in an ionization chamber, upstream of the "nozzles", by a high voltage corona discharge and is then transported to and through the "nozzles", where it is electrically controlled by electrical potentials applied to an array of marking elements, in the form of modulation electrodes, one associated with each "nozzle". Selective control of the electrical potential applied to each of the modulation electrode in the array will enable areas of charge and areas of absence of charge to be deposited on the receptor surface for subsequently being made visible by suitable development apparatus. A marking head of page width, i.e., about 8.5 inches wide, having a resolution of 200 to 400 spots per inch (spi) would result in an array of 1700 to 3400 modulation electrodes. Typically, for a 300 spi writing head, each of the modulating electrodes would be about 2.3 mils wide and have an interelectrode spacing of about 1 mil. The head array is divided into a number of sections of the modulation electrodes, arranged so that each section may be sequentially isolated and addressed by a compact, multiplexed, data loading circuit, integrated upon the head array substrate for bringing each of the modulation electrodes to the desired voltage (0 volts for "writing" or 10 to 30 volts for "non-writing"). Gray scale also may be achieved by imposing intermediate modulation voltage values on the modulating electrodes, for placing intermediate charge values upon the receptor surface which, when developed, exhibit a range of optical densities.
In U.S. Pat. No. 4,584,522 the modulation electrodes in each selected section are rapidly brought to the predetermined control voltage when coupled to data buses during a short segment of the entire line writing time. After being loaded each section is decoupled from the data buses and each modulation electrode will hold its applied voltage ("float") for the remainder of the line writing time. Typically, loading of each section can be accomplished in about 2.5% of the line writing time, allowing the modulation electrode to float for about 97.5% of the line writing time, until it is again addressed. The data loading circuit in U.S. Pat. No. 4,719,481 allows the modulation electrodes in each selected section to be directly connected to either a source of writing potential or a source of non-writing potential, each being supplied by a suitable bus line. In practice, the electrodes are held at either a reference (i.e. ground) potential, or a higher (15 to 30 volts) potential, respectively. While there are certain advantages to be derived from always maintaining the correct potential on the modulation electrodes, a disadvantage is that marking latitude is limited because it is not possible to apply a potential of any desired intermediate value as is necessary for gray scale marking.
In high humidity conditions, e.g., RH&gt;50%, we have observed the occurrence of image blurring, or smearing, during operation of the marking device incorporating the head array of the U.S. Pat. No. 4,584,522 type. This has been attributed to interpixel current leakage. It was further observed during testing, that image blurring did not occur until after the marking head array had been exposed to corona effluents.
It is the primary object of this invention to provide an improved marking head array in which marking will be virtually unaffected by high humidity conditions.
It is another object of this invention to provide an improved marking head array which is heated for eliminating moisture collection upon its surface.